Numerical control apparatuses (CNCs) are used at each station of a transfer line for machining parts for automobiles, and these CNCs contain a programmable machine controller (PMC) for controlling only a single axis. The CNCs of this type are referred to as a single-axis CNCs.
Although these single-axis CNCs are usually independently controlled, sometimes a plurality of single-axis CNCs must be simultaneously controlled for performing various operations, i.e., cutting an arc, and to accomplish this, the I/O circuits of the respective CNCs are interconnected and data is exchanged through the I/O circuits.
Nevertheless, when data is exchanged through the I/O circuits, in the operation of the respective CNCs, a time lag occurs and further, communication therebetween becomes complicated, and as a result, a high accurate synchronized operation, such as an interpolation, cannot be carried out.